There was but one limitation to the universal liability of slaves. They could not be tortured to extract testimony against their masters, whether in civil or criminal cases;[1415] though, if a slave had been purchased by a litigant to get his testimony out of court, the sale was pronounced void, the price was refunded, and the slave could then be tortured.[1416] This limitation arose from a careful regard for the safety of the master, and not from any feeling of humanity towards the slave. So great a respect, indeed, was paid to the relationship between the master and his slave that the principle was pushed to its fullest extent. Thus even an employer, who was not the owner of a slave, was protected against the testimony of the latter.[1417] When a slave was held in common by several owners, he could not be tortured in opposition to any of them, unless one were accused of murdering his partner.[1418] A slave could not be tortured in a prosecution against the father or mother of the owner, or even against the guardian, except in cases concerning the guardianship;[1419] though the slave of a husband could be tortured against the wife.[1420] Even the tie which bound the freedman to his patron was sufficient to preserve the former from being tortured against the latter;[1421] whence we may assume that, in other cases, manumission afforded no protection from the rack and scourge. This question, however, appears doubtful. The exemption of freedmen would seem to be proved by the rescript which provides that inconvenient testimony should not be got rid of by manumitting slaves so as to prevent their being subjected to torture;[1422] while, on the other hand, a decision of Diocletian directs that, in cases of alleged fraudulent wills, the slaves and even the freedmen of the heir could be tortured to ascertain the truth.[1423]